Explanation of 555 astable circuit

I'm trying to get into electronics and I'm starting with a simple 555 timer circuit. I'm reading up on general circuit design but I find most websites tend to just give circuit diagrams with no explanation of the purpose of the components. I've not got any intuition for the various components yet so things are a bit confusing.

I want a 50% duty cycle. I can see examples with a diode across R2 and presumably with R1 and R2 equal it would be 50% (more or less). With my limited knowledge though I wonder why two resistors are required in that case (ie. why not remove R1 and the diode). The only reason I can think of is that R1 is doing something else in the circuit other than affecting the charge/discharge time. I've also read that it's not accurate to use a single resistor. Could anyone enlighten me as to why I need R1? Or is there a good resource that explains the 555 astable circuit thoroughly?